1. Field of the Invention
This invention relates to a flat image display device provided with a pair of substrates opposed to each other.
2. Description of the Related Art
Various flat image display devices have been developed as a next generation of image display devices in which a large number of electron emitting elements are arranged side by side and opposed to a phosphor screen. While there are various types of electron emitting elements for use as electron emission sources, all of them basically utilize field emission. Display devices that use these electron emitting elements are generally called field emission displays (FED's). Among the FED's, a display device that uses surface-conduction electron emitting elements is also called a surface-conduction electron emission display (SED). In this specification, however, the term “FED” is used as a generic name for devices including the SED.
In general, an FED comprises a first substrate and a second substrate that are opposed to each other with a given gap between them. These substrates have their respective peripheral portions joined together by a sidewall in the shape of a rectangular frame, thereby constituting a vacuum envelope. The interior of the vacuum envelope is kept at a high vacuum such that the degree of vacuum is about 10−4 Pa or below. In order to support an atmospheric load that acts on the first substrate and the second substrate, a plurality of support members are located between these substrates.
A phosphor screen that includes red, blue, and green phosphor layers is formed on the inner surface of the first substrate, and a large number of electron emitting elements that emit electrons for exciting the phosphor to luminescence are provided on the inner surface of the second substrate. Further, a large number of scan lines and signal lines are formed in a matrix and connected to the electron emitting elements. An anode voltage is applied to the phosphor screen, and electron beams emitted from the electron emitting elements are accelerated by the anode voltage and collide with the phosphor screen, whereupon the phosphor glows and displays an image.
In the FED of this type, the gap between the first and second substrates can be set to several millimeters or less. When compared with a cathode-ray tube (CRT) that is used as a display of an existing TV or computer, therefore, the FED can achieve lighter weight and smaller thickness.
In order to obtain practical display characteristics for the FED constructed in this manner, it is necessary to use a phosphor that resembles that of a conventional cathode-ray tube and to use a phosphor screen that is obtained by forming a thin aluminum film called a metal back on the phosphor. In this case, the anode voltage to be applied to the phosphor screen should be set to at least several kV, and preferably, to 10 kV or more.
In view of the resolution, the properties of the support members, etc., the gap between the first substrate and the second substrate cannot be made very wide and should be set to about 1 to 2 mm. In the FED, therefore, a strong electric field is inevitably formed in the narrow gap between the first substrate and the second substrate, so that electric discharge (dielectric breakdown) between the substrates raises a problem.
If electric discharge occurs, a current of 100 A or more sometimes may flow instantaneously, so that the electron emitting elements and the phosphor screen may be broken or degraded, and moreover, a driver circuit may possibly be broken. These failures will be referred to collectively as electric discharge damage. Electric discharge that results in these failures is not allowed for products. In order to put the FED into practical use, it must be constructed so that it can be prevented from being damaged by electric discharge for a long period of time. It is very hard, however, to restrain electric discharge perfectly for a long period of time.
Supposedly, a measure may be taken to suppress the scale of electric discharge so that the influence of occurrence of electric discharge, if any, on the electron emitting elements, phosphor screen, and driver circuit is negligible, not to prevent generation of the electric discharge. A technique associated with this idea is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2000-311642, for example. According to this technique, a zigzag pattern or the like is formed by notching a metal back on a phosphor screen, whereby the effective inductance and resistance of the phosphor screen is enhanced. Disclosed in Jpn. Pat. Appln. KOKAI Publication No. 10-326583, on the other hand, is a technique in which a metal back is divided and connected to a common electrode through a resistance member so that a high voltage can be applied.
It is difficult, however, for even these techniques to fully restrain electric discharge damage to the phosphor screen and the electron emitting elements. There is another technique in which electric discharge is restrained by providing a high-resistance version of metal back. If the resistance of the metal back is enhanced, however, the metal back becomes transparent and inevitably ceases to fulfill its function.